The SMPTE 259M has hitherto been known as the standard for serial digital interface (hereinafter simply referred to as a "SDI") of video signal used in professional and broadcasting station video equipments. This SMPTE 259M standard describes a serial digital interface for 525/60 and 625/50 digital television equipment operating with either 4:2:2 component signals or 4f.sub.sc composite digital signals.
FIG. 1 of the accompanying drawings shows the SDI format.
As shown in FIG. 1, the SDI format comprises an EAV (end of active video) 131 provided at the starting portion, an ancillary signal area 132 provided next to the EAV 131, and a SAV (start of active video) 133 provided next to the ancillary signal area 132. The EAV 131 and SAV 133 comprise words (3FF, 000, 000, XYZ).sub.h each word of which is a hexadecimal signal. The EAV 131, the ancillary signal area 132, and the SAV 133 are composed of 276 words for the 525/60 television receiver, and composed of 288 words for the 625/50 the television receiver.
The SDI format further includes an effective image signal area 134 provided next to the SAV 133, and the effective image signal area 134 is used to transmit an image signal. The effective image signal area 134 is followed by a timing reference signal EAV 135. The image signal comprises a luminance signal Y and blue and red color difference signals Cb, Cr each of which is a 10-bit digitized signal, and the luminance signal Y and the blue and red color difference signals are arranged in the sequential order of Cb, Y, Cr and Y. The effective image signal area 134 is composed of 1440 words for both the 525/60 television receiver and the 625/50 television receiver. Therefore, an area which results from adding the effective image signal area 134 to the EAV 131, the ancillary signal area 132 and the SAV 133 is composed of 1716 words for the 525/60 television receiver, and composed of 1728 words for the 625/50 television receiver.
The effective image signal area 134 is followed by the timing reference signal EAV 135. The timing reference signal EAV 135 is composed of words (3FF, 000, 000, XYZ).sub.h each word of which is a hexadecimal signal. The SAV 133, and the EAV 131, 135 are inserted into the horizontal blanking period.
A parallel signal according to the SDI format is converted in the form of parallel to serial signal, and in this case, the LSB (least significant bit) of any data word shall be transmitted first. Then, the serial signal is converted into a scrambled NRZI (nonreturn-to-zero inverted) signal, channel-coded, and thereby transmitted as serial digital video signal of 270 Mbit/sec.
In the SDI format signal transmission system, the serial digital video signal is transmitted with a word synchronization of word unit. However, in this SDI format signal transmission system, it is frequently demanded that a plurality of signals are switched by a signal switching device. This signal switching device switches and outputs the inputted signal in the form of the serial signal. Before or after the signal is switched by the signal switching device, a word synchronization of signal becomes discontinuous, and a synchronization of switched signal is not achieved, thereby causing a transmission error. The transmission error continuously occurs until a word synchronization of signal is established by the next EAV.
For this reason, according to the SDI format, a signal is switched during a vertical blanking period, whereby a signal error that occurred due to a discontinuous word synchronization of signal generated when the signal is switched can be prevented from becoming conspicuous on a picture screen of a television receiver on the reception side.
As described above, in the SDI format signal transmission system, before or after a plurality of signals are switched by the signal switching device, a word synchronization of signal becomes discontinuous, and a synchronization of switched signal cannot be established, resulting in a transmission error. In order to avoid the occurrence of transmission error, a signal has to be switched during the vertical blanking period.